Re: io_uring and spurious wake-ups from eventfd

[Mark Papadakis <markuspapadakis@icloud.com>]

> On 7 Jan 2020, at 10:34 PM, Jens Axboe <axboe@kernel.dk> wrote:
> 
> On 1/7/20 1:26 PM, Jens Axboe wrote:
>> On 1/7/20 8:55 AM, Mark Papadakis wrote:
>>> This is perhaps an odd request, but if it’s trivial to implement
>>> support for this described feature, it could help others like it ‘d
>>> help me (I ‘ve been experimenting with io_uring for some time now).
>>> 
>>> Being able to register an eventfd with an io_uring context is very
>>> handy, if you e.g have some sort of reactor thread multiplexing I/O
>>> using epoll etc, where you want to be notified when there are pending
>>> CQEs to drain. The problem, such as it is, is that this can result in
>>> un-necessary/spurious wake-ups.
>>> 
>>> If, for example, you are monitoring some sockets for EPOLLIN, and when
>>> poll says you have pending bytes to read from their sockets, and said
>>> sockets are non-blocking, and for each some reported event you reserve
>>> an SQE for preadv() to read that data and then you io_uring_enter to
>>> submit the SQEs, because the data is readily available, as soon as
>>> io_uring_enter returns, you will have your completions available -
>>> which you can process.  The “problem” is that poll will wake up
>>> immediately thereafter in the next reactor loop iteration because
>>> eventfd was tripped (which is reasonable but un-necessary).
>>> 
>>> What if there was a flag for io_uring_setup() so that the eventfd
>>> would only be tripped for CQEs that were processed asynchronously, or,
>>> if that’s non-trivial, only for CQEs that reference file FDs?
>>> 
>>> That’d help with that spurious wake-up.
>> 
>> One easy way to do that would be for the application to signal that it
>> doesn't want eventfd notifications for certain requests. Like using an
>> IOSQE_ flag for that. Then you could set that on the requests you submit
>> in response to triggering an eventfd event.
> 


Thanks Jens,

This is great, but perhaps there is a somewhat slightly more optimal way to do this.
Ideally, io_uring should trip the eventfd if there are any new completions available, that haven’t been produced
In the context of an io_uring_enter(). That is to say, if any SQEs can be immediately served (because data is readily available in
Buffers/caches in the kernel), then their respective CQEs will be produced in the context of that io_uring_enter() that submitted said SQEs(and thus the CQEs can be processed immediately after io_uring_enter() returns). 
So, if any CQEs are placed in the respective ring at any other time, but not during an io_uring_enter() call, then it means those completions were produced asynchronously, and thus the eventfd can be tripped, otherwise, there is no need to trip the eventfd at all.

e.g (pseudocode):
void produce_completion(cfq_ctx *ctx, const bool in_io_uring_enter_ctx) {
        cqe_ring_push(cqe_from_ctx(ctx));
        if (false == in_io_uring_enter_ctx && eventfd_registered()) {
                trip_iouring_eventfd();
        } else {
                // don't bother
        }
}

@markpapadakis